Several effects of melt viscosity on the nature ofC _p —T data aboveT _g for acrylate, methacrylate, and hydrocarbon polymers

Abstract

Using objective computerized statistical procedures, we have examined high precisionC _p data by DSC reported by Wunderlich and Gaur for a series of alkyl acrylate and methacrylate polymers. Although they claimed the data to be linear inT aboveT _g , our results do not support the linear model. One or two endothermic slope changes are revealed aboveT _g in lowT _f polymers (T _f < 20 °C) and at least one exothermic slope change in highT _f polymers (T _f > 20 °C).T _f is the flow temperature of Ueberreiter. Both the first endotherm and the first exotherm occur near (1.22 ± 0.07)T _g , suggesting aT _ll type phenomenon.T _ll varies as$1/\bar M_n$. The first exotherm is associated by us with wetting of the DSC pan by molten polymer on the first heating of particulate highT _f polymers. The rate of wetting, and presumably the magnitude of the exotherm, depends in part on the ratio,γ/η, whereγ is surface tension andη is melt viscosity of the molten polymer. Sinceγ is relatively constant, the molecular weight and temperature dependence for rate of wetting resides inη, which depends on$\bar M_w$. For$\bar M_n > > \bar M_c$, a second exothermic event caused by sintering, and also controlled by η, may be present. The interactive roles of$\bar M_n ,\bar M_w ,\bar M_w /\bar M_n$ ;M _c (entanglement molecular weight); particle size, and heating rate onC _p —T behaviour are delineated for the first time. LowT _f hydrocarbon polymers, namely atactic polyalphaolefins,C ₃ ,C ₅ ,C ₆ ; PIB; and dienes, PBD and cis-PI, exhibit single or double endotherms. Other results on highT _f polymers showing exothermic effects, notably PS, PnBMA and polyglycidylmethacrylate are cited.